Three-dimensional printing is used to fabricate relatively small objects for prototyping and custom-manufacturing in a variety of fields. Three-dimensional printing processes typically fabricate an object layer upon layer, such that each sequential layer is shaped according to a respective cross section of the object.
Some objects that are to be fabricated by three-dimensional printing incorporate overhanging regions that need to be supported during fabrication. In such cases, a three-dimensional printing process simultaneously fabricates both an object body structure and a support structure, any of which may be present in each layer. The support structure is removed after fabrication of the object is complete.
Three-dimensional printing technologies of special interest include selective deposition or selective curing.
In selective deposition, a computer-controlled liquid dispenser, such as an inkjet printing head, deposits a new layer of liquid material upon a surface, e.g. a previously-deposited cross section layer. Each layer includes an object body cross section and, if necessary, a support cross section. “Selective deposition” is so-named because the dispenser accurately places tiny droplets of material according to a two-dimensional image of the object body cross section, and, if needed, tiny droplets of support material according to the support structure cross section. For the body of the fabricated object, the deposited material is typically a photopolymer that is solidified by being irradiated with curing radiation during the printing process. The curing radiation is usually in the ultraviolet portion of the spectrum, for solidifying the polymer. Alternatively, a deposited material, such as a wax, may be liquified by heat for deposition, and allowed to solidify by natural cooling after being deposited in place. The term “curing” herein denotes a chemical process by which a liquid photopolymer is solidified.
In selective curing, a high-resolution image of the two-dimensional object body cross section is projected by an image projector onto a thin, uniform layer of liquid photopolymer, thereby selectively solidifying the projected cross section. Non-limiting examples of image projectors include: Digital Light Processing (DLP) projectors using UV sources; and scanning UV lasers. “Selective curing” is so-named because the cross section is shaped by accurately curing only those portions which are to be solidified according to the desired cross section, for example by UV curing as just described. In some known methods of selective curing, the current cross section layer is added at the top of the object body being fabricated, at the surface of a liquid polymer in a trough, or alternatively at the bottom of an object body being fabricated and which is suspended in a trough with a transparent bottom, through which the cross section layer image is projected for curing a thin layer of liquid at the bottom. Where supporting structures are necessary to suspend overhanging or hollow parts of the object body, these are implemented as thin strings or pillars of build material selectively cured during the same printing process. The support strings are broken and removed after fabricating the body is completed.
Selective deposition allows the use of multiple materials with different properties, which enables fabricating:                support structures that are easy-to-remove by dissolution or melting, and        parts of the object body with diverse colors and/or mechanical properties and/or other properties.        
Selective curing offers advantages including:                faster printing; and        allowing the use of slurries of solid particles (such as particles of metal or ceramic) suspended in liquid polymer, which cannot be accurately dispensed via the printing heads used for selective deposition.        
Selective curing, however, currently has certain limitations, notably the limitation of being able to use only a single material.
The techniques and components for selective deposition and selective curing as described herein for use in embodiments of the present invention are well-known in the field, and are described herein for purposes of reference.